Polysaccharides possess remarkable immune-modulatory activities, making them valuable for development in both the food and pharmaceutical industries. Currently, there is a significant amount of research focusing on the chemical structures and immune activities of polysaccharides. The structure-activity relationship (SAR) of polysaccharides refers to the correlation between their physical and chemical properties, primary structure, and higher-order structure with their pharmacological activities. The biological functional activities of polysaccharides are determined by factors such as their molecular weight, structure, and spatial conformation. Furthermore, alterations in the complex structures of polysaccharides, including the linkage sequence of isomeric glycosyl units, the position of substituents, the flexibility of the main chain, and helical conformations, can also lead to changes in their activities. Molecular modification is one of the important approaches to studying the SAR of polysaccharides.

1. The Relationship Between Physical Properties and the Immunomodulatory Activity of Polysaccharides

Physical properties primarily refer to the viscosity, solubility, and molecular weight of polysaccharides. Reducing the viscosity and enhancing the water solubility of polysaccharides often lead to an enhancement of their biological activities. Generally speaking, degrading polysaccharides with higher molecular weights into those with lower molecular weights can significantly improve their pharmacological activities. However, it is not the case that lower molecular weights are always better. Studies have shown that polysaccharides with excessively low relative molecular masses cannot form active polymeric structures, while excessively high relative molecular masses hinder polysaccharides from crossing cell membranes to exert their activities within the body. Different polysaccharides exhibit optimal ranges of relative molecular masses for exerting their immunomodulatory activities. For instance, in an evaluation using the RAW264.7 macrophage model to assess the impact of carboxymethyl pachyman (CMP) with various relative molecular masses on the release of nitric oxide (NO) and tumor necrosis factor-alpha (TNF-α), all CMP samples with different relative molecular masses were found to stimulate immune responses in RAW264.7 macrophages, with medium-sized CMP exhibiting the strongest activity, while low-molecular-weight CMP demonstrated the weakest. In an LPS-induced RAW264.7 inflammation model, higher relative molecular masses of CMP showed stronger inhibitory effects on the synthesis of NO and TNF-α.

2. The Relationship Between Glycosyl Composition, Glycosidic Bond Types, and the Immunomodulatory Activity of Polysaccharides

Polysaccharides composed of different types of monosaccharides and glycosidic bonds exhibit significant variations in their biological activities. The type and position of glycosidic bonds can influence the anti-inflammatory activity of polysaccharides, which is intimately linked to the immune regulation of the organism. Studies on the in vitro immunomodulatory activity of polysaccharides extracted from shiitake mushrooms at different developmental stages of their fruiting bodies revealed that polysaccharides from young mushrooms exhibit the strongest immune activity. The higher proportions of galactose and mannose in their monosaccharide composition compared to other stages suggest that glycosyl composition is one of the primary factors contributing to their heightened immune activity.

Research has found that most polysaccharides with immunomodulatory activity contain galactose, glucose, arabinose, and mannose, along with other sugars such as galacturonic acid, rhamnose, xylose, fucose, fructose, and glucuronic acid. For example, Zhishi polysaccharides consist of galactose, glucose, arabinose, and mannose, while Jinyingzi polysaccharides contain galactose, glucose, arabinose, mannose, rhamnose, xylose, and galacturonic acid, with a rich abundance of arabinose and glucose. Polysaccharides rich in these sugars tend to exhibit better immune effects. In the case of Zhishi polysaccharides CAVAP-Ⅰ and CAVAP-Ⅱ, both are composed of the same types of monosaccharides but differ significantly in their content. CAVAP-Ⅱ, with a significantly higher content of arabinose than CAVAP-Ⅰ, demonstrates superior immune-enhancing activity compared to CAVAP-Ⅰ.

The Glycosidic Bonds Between Polysaccharide Glycosyls Play a Decisive Role in the Local Conformation of Polysaccharide Molecular Chains, Influencing Their Spatial Morphological Structures, and Thus, the Type of Glycosidic Bond Has a Crucial Impact on the Immunomodulatory Activity of Polysaccharides.The glycosidic bonds between polysaccharide glycosyls determine the local conformation of polysaccharide molecular chains, which can affect their spatial morphological structures. Consequently, the type of glycosidic bond exerts a significant influence on the immunomodulatory activity of polysaccharides. Most polysaccharides with robust immunomodulatory activities are associated with β-(1→3) glycosidic bonds. For instance, β-(1→3)-D-Glc plays a pivotal role in the immune activity of polysaccharides from five edible mushrooms, including oyster mushroom, agrocybe aegerita, shiitake mushroom, black fungus, and enokitake mushroom. As the content of β-(1→3)-D-Glc increases, the immune activity of these five polysaccharides enhances. The main chain of shiitake mushroom polysaccharide, which is composed of glucans linked by (1→3) glycosidic bonds, exhibits potent immune activity. In contrast, starch, whose main chain is glucan linked by (1→4) glycosidic bonds, lacks immunobiological activity. Furthermore, polysaccharides with a β-(1→3) glycosidic bond in the main chain and β-(1→6) glycosidic bonds in the side chains also demonstrate good immune activity. Additionally, the immune activity of some polysaccharides is closely related to (1→4) glycosidic bonds.

3. The Relationship Between Substituent Groups and the Immunomodulatory Activity of Polysaccharides

The presence or absence of substituent groups and their types significantly impact the immunomodulatory activity of polysaccharides. Substituent groups can be added or removed through chemical methods, such as sulfation, acetylation, carboxymethylation, and phosphorylation, with sulfation being the most common modification that holds great significance for the development and utilization of polysaccharides.

① Sulfation: The sulfation modification of polysaccharides primarily involves introducing sulfate groups into polysaccharide molecules through the chlorosulfonic acid-pyridine method. This modification alters the structure and conformation of polysaccharides, thereby changing their biological activities. Numerous experimental studies have demonstrated that sulfated polysaccharides exhibit enhanced immunomodulatory activity. For instance, sulfation of yam polysaccharide (S-CYP) enhances splenic lymphocyte proliferation, promotes the release of cytokines like TNF-α and IL-1β, and stimulates the production of IgG and IgM in serum, thereby improving the immunomodulatory activity of yam polysaccharide. Similarly, sulfation of Lycium barbarum polysaccharides (S-LbGp1, S-LbGp2) significantly enhances their immunomodulatory activity, manifested by a more pronounced stimulation of RAW264.7 cell proliferation, more effective promotion of cytokine secretion, and improved induction of NO production in LbGp4-stimulated cells, resulting in increased intracellular acid phosphatase activity in macrophages.

② Carboxymethylation: The carboxymethylation of polysaccharides is typically achieved by reacting polysaccharides with chloroacetic acid to yield carboxymethylated polysaccharides, introducing carboxymethyl groups into the polysaccharide molecules. This modification alters the structure of polysaccharides, leading to enhanced immunomodulatory activity. Studies have found that carboxymethylated Schisandra chinensis polysaccharide (CSPP) exhibits a random coil structure in aqueous solution, possessing good solubility. Compared to unmodified polysaccharides, CSPP displays higher immunomodulatory activity in mice exposed to polychlorinated biphenyl 126 (PCB126). Researchers have investigated the immunomodulatory activity of crude polysaccharides from Rhizopogon rubescens and carboxymethylated Rhizopogon rubescens acid polysaccharides in mice. The results indicate that carboxymethylated Rhizopogon rubescens acid polysaccharides possess stronger immunomodulatory activity.

③ Acetylation: Upon acetylation, polysaccharides exhibit exposed hydroxyl groups, which increase their solubility in water, favoring the enhancement of immunomodulatory activity. Research on the effects of Cyclocarya paliurus polysaccharide (CPP0.1) and acetylated Cyclocarya paliurus polysaccharide (Ac-CPP0.1) on the immune organ indices and cytokines of immunosuppressed mice revealed that both CPP0.1 and Ac-CPP0.1 possess immunomodulatory effects, with Ac-CPP0.1 demonstrating the highest immunomodulatory activity. Further evaluation of the immunomodulatory activity of Abelmoschus manihot stem and leaf polysaccharide (SLAMP-a) and three acetylated variants (Ac-SLAMP-a1, Ac-SLAMP-a2, Ac-SLAMP-a3) using in vitro splenocyte proliferation and NO release by RAW264.7 cells as indicators showed that SLAMP-a lacked immunomodulatory activity, whereas Ac-SLAMP-a1 significantly improved in vitro immunomodulatory activity. Additional studies indicated that acetylated Atractylodes lancea polysaccharide exhibited improved solubility compared to unmodified, carboxymethylated, and phosphorylated forms. Among these modifications, acetylated Atractylodes lancea polysaccharide elicited the strongest ability to stimulate RAW264.7 macrophages to release TNF-α and IL-6, thus enhancing its immunomodulatory activity.

④ Selenization: Selenium can be chemically bound to polysaccharides to form selenium-polysaccharides, which are characterized by low side effects and high bioavailability. Selenium-polysaccharides generally exhibit higher immunomodulatory activity than selenium or polysaccharides alone and are more readily absorbed and utilized by the body. Research has shown that selenized Rosa laevigata fruit polysaccharide (Se-PPRLMF-2) enhances the pinocytosis and phagocytic capabilities of RAW264.7 cells, promoting the secretion of cytokines (TNF-α, IL-6, and NO), thereby significantly improving its immunomodulatory activity. Selenium-acidified Atractylodes macrocephala polysaccharide (sAMP) was synthesized using the HNO3-Na2SeO3 method, and its effects on immune organ indices, phagocytic coefficients, cytokines, and splenocyte proliferation in cyclophosphamide-induced immunocompromised mice were measured, with unmodified AMP and sodium nitrite as controls. The results indicated that selenization enhanced the immunomodulatory activity of AMP. Additionally, studies found that selenized Ganoderma lucidum mycelium polysaccharide (SeMPN) significantly increased the pinocytosis and phagocytic abilities of RAW264.7 cells, as well as the production of NO, TNF-α, and IL-6.

4. Relationship between the Advanced Structure and Pharmacological Activity of Polysaccharides

The advanced structure of polysaccharides exerts a greater influence on their immunomodulatory activity than their primary structure. The known conformations of polysaccharides include irregular coils, single helices, double helices, triple helices, rod-like, worm-like, spherical, and others. These structural features affect the recognition of polysaccharides by immune cells, thereby influencing their immunomodulatory activity. Different conformations of polysaccharides lead to distinct activities due to variations in linkage patterns, branching structures, degrees of branching, electrostatic repulsion between intermolecular hydrogen bonds and substituents. Polysaccharides with immunomodulatory activity predominantly adopt a triple helix conformation, which forms a rigid tertiary structure, exhibiting stronger biological activity. They can exert immunomodulatory effects by enhancing cellular phagocytosis and proliferation, activating corresponding signal transduction pathways, and increasing the secretion of cytokines.

References:

[1] Tan Xi, Zhou Xin, Chen Huaguo. "Research Progress on the Structure-Activity Relationship of Plant Polysaccharides." China Journal of Chinese Materia Medica, 2017, 42(21): 4104-4109.

[2] Shang Tingting, Li Tianfeng, Zhou Jing, et al. "Research Progress on the Structure-Activity Relationship of Plant Polysaccharides." Guangdong Chemical Industry, 2019, 46(08): 99-100+122.

[3] Chen Nuo, Xi Wenjie, Hu Meifen, et al. "Research Progress on the Relationship between the Immunomodulatory Effects and Structures of Polysaccharides." China Journal of Chinese Materia Medica, 2023, 48(10): 2667-2678.

About the Author

Xiaonisha, a food technology professional holding a Master's degree in Food Science, is currently employed at a prominent domestic pharmaceutical research and development company. Her primary focus lies in the development and research of nutritional foods, where she contributes her expertise and passion to create innovative products.